The term "stent" is intended to indicate in general a device to be fitted in a lumen (for example, inside a blood vessel), usually by catheterization, and subsequently spread out in situ in order to support the lumen locally. This has the main purpose of preventing the re-establishment of a stenotic site in the location treated. It should, however, be pointed out that it has already been proposed in the art to use substantially similar structures for spreading-out and anchoring vascular grafts in situ; naturally this possible extension of the field of application is also intended to be included in the scope of the invention.
For a general teaching with regard to vascular stents, reference may usefully be made to the work "Textbook of Interventional Cardiology" by Eric J. Topol, W. B. Saunders Company, 1994 and, in particular, to Section IV of Vol. II, entitled "Coronary stenting".
A large number of patent documents are also dedicated to the subject as is shown, for example, by U.S. Pat. No. 4,776,337, U.S. Pat. No. 4,800,882, U.S. Pat. No. 4,907,336, U.S. Pat. No. 4,886,062, U.S. Pat. No. 4,830,003, U.S. Pat. No. 4,856,516, U.S. Pat. No. 4,768,507, and U.S. Pat. No. 4,503,569.
In spite of extensive research and experimentation as documented at the patent level, only a very small number of operative solutions has up to now been used in practice.
This fact can be attributed to various factors, amongst which the following problems or requirements may be mentioned:
to ensure that, during its advance towards the site to be treated, the stent can adapt in a sufficiently flexible manner to the path along which it is travelling even with regard to portions having small radii of curvature such as those which may be encountered, for example, in some peripheral vessels; this must be achieved without adversely affecting the ability of the stent to perform an effective supporting action once positioned and spread out, PA1 to prevent, or at least limit the effect of longitudinal shortening which occurs in many stents when they are spread out, PA1 to offer as broad as possible a bearing surface to the wall of the lumen to be supported, PA1 to avoid giving rise to complex geometry and/or to possible stagnation sites which, particularly in applications in blood vessels, may give rise to adverse phenomena such as coagulation, clotting, etc., and PA1 to reconcile the requirements set out above with simple and reliable production methods and criteria, within the scope of currently available technology. PA1 the body comprises a plurality of generally annular segments, the wall of each segment being defined by a plurality of loops, and PA1 at least some of the segments are interconnected by bridge elements extending in the general direction of the longitudinal axis of the stent and having at least one end connected to the concave or inside portion of a respective loop so that, during the radial expansion of the stent, the axial contraction of the segments resulting from the opening-out of the respective loops is compensated by axial projection of the bridge elements from the respective concave portions. PA1 the body comprises a plurality of successive radially expandable segments interconnected by bridge elements extending substantially in the direction of the longitudinal axis of the stent so that the bridge elements are substantially unaffected by the radial expansion of the segments and the bridge elements are generally deformable in the direction of the longitudinal axis so that the length of the stent along the axis can change substantially independently of the radial expansion.